Under the pressure of increasingly stricter environmental legislation, the development of powder lacquers, in addition to high-solids lacquers and aqueous coating systems, has become increasingly important in recent years. Powder lacquers do not release any harmful solvents during their application, can be processed with very high material efficiency and recovery, and are therefore regarded as particularly environmentally friendly and economical.
Extremely high-grade, light-resistant and weather-resistant coatings can be produced with heat-curable powder lacquers based on polyurethane. The polyurethane (PUR) powder lacquers currently established in the market generally consist of solid polyester polyols, which are hardened (cured) with solid blocked aliphatic or in most cases cycloaliphatic polyisocyanates. These systems have the disadvantage, however, that under thermal cross-linking the compounds used as blocking agents split off and largely escape. Accordingly, when they are processed, for apparatus reasons as well as for reasons of ecology and work safety special measures have to be taken to purify the waste air and/or to recover the blocking agent.
One possible way of preventing the emission of blocking agents is to use the known PUR powder lacquer cross-linking agents containing uretdione groups (e.g. DE-A-2 312 391, DE-A 2 420 475, EP-A 0 045 994, EP-A 0 045 996, EP-A 0 045 998, EP-A 0 639 598 or EP-A 0 669 353). In these products, the thermal reverse cleavage of uretdione groups into free isocyanate groups and their reaction with the hydroxyfunctional binding agent is used as the cross-linking principle. In practice uretdione powder lacquer cross-linking agents have, however, up to now been used only to a small extent. The reason for this is the comparatively low reactivity of the internally blocked isocyanate groups, which as a rule requires stoving temperatures of at least 160° C.
Although it is known that the cleavage of uretdione groups occurs noticeably starting at about 100° C., especially in the presence of reactants containing hydroxyl groups, the reaction still proceeds so slowly in this temperature range that unrealistically long times of several hours are required for the complete hardening of lacquer films for practical use. Although in DE-A 2 420 475, DE-A 2 502 934 or EP-A 0 639 598 temperatures starting from 110° C. are already given as possible stoving conditions for powder lacquer systems containing uretdione groups, and in DE-A 2 312 391 even temperatures starting from 90° C. are mentioned, the specifically described embodiments show sufficiently cross-linked coatings can be obtained under practical stoving times of at most 30 minutes only starting at temperatures of 150° to 160° C. These publications do not disclose how powder lacquers can be prepared that can actually be completely hardened to a commercially usable extent at temperatures below 150° C. to 160° C.
There has been no lack of attempts to accelerate the hardening of uretdione-cross-linked PUR powder lacquers by the combined use of suitable catalysts. Various compounds have been suggested for this purpose, for example, the organometallic catalysts known from polyurethane chemistry, such as tin(II) acetate, tin(II) octoate, tin(II) ethylcaproate, tin(II) laurate, dibutyltin diacetate, dibutyltin dilaurate, dibutyltin maleate, (e.g. EP-A 0 045 994, EP-A 0 045 998, EP-A 0 601 079, WO 91/07452 or DE-A 2 420 475), iron(III) chloride, zinc chloride, zinc-2-ethylcaproate and molybdenum glycolate, tertiary amines such as triethylamine, pyridine, methylpyridine, benzyldimethylamine, N,N-endoethylenepiperizine, N-methylpiperidine, pentamethyldiethylenetriamine, N,N′-dimethyl-piperazine (e.g. EP-A 0 639 598) or N,N,N′-trisubstituted amidines, in particular bicyclic amidines such as 1,5-diazabicyclo[4.3.0]non-5-ene(DBN) (e.g. EP-A 0 803 524).
Of these catalysts, the aforementioned bicyclic amidines allow the lowest stoving temperatures. At the same time, however, they lead to a stoving yellowing that is unacceptably high for many areas of application. For this reason amidine-catalysed uretdione systems have not been able to penetrate the market.
Recently further progress has been achieved in lowering the stoving temperature of uretdione powder lacquers.
According to the teaching of EP-A 1 137 689, Lewis acid catalysts, such as for example the aforementioned tin or zinc compounds, are inhibited by acidic groups, such as carboxyl groups. Their full catalytic activity can therefore be displayed in a uretdione powder lacquer system only if the employed hydroxyfunctional binder is free of carboxyl groups. For this reason a sufficient amount of an agent reactive with respect to carboxyl groups, for example an epoxide, is added to the powder lacquers described in this publication, which consist of conventional hydroxyfunctional binders, cross-linking agents containing uretdione groups and special Lewis acid catalysts, in order to convert as completely as possible any carboxyl groups possibly still present in the binder and thereby remove them from the system. In this way the reactivity of the polyurethane powders can in fact be increased to such an extent that the hardening already commences starting at a temperature of ca. 120° C., though the lacquer films obtained thereby exhibit a completely unsatisfactory flow behaviour, which is reflected in a marked surface structure and absence of gloss. Although improved lacquer formulations of this type are described in WO 2005/095482, which in the presence of zinc catalysts cross-link at low temperatures and provide coatings having an acceptable flow, these formulations involve the use of very special hydroxyl functional binders with an exactly defined residual content of acidic groups.
Further catalysts, with which, in the absence of carboxyl groups, and with the combined use of a compound reactive with respect to carboxyl groups, the stoving temperature of uretdione powder lacquers can similarly be significantly reduced, as with the Lewis acid catalysts of EP-A 1 137 689, are for example the ammonium hydroxides and fluorides described in EP-A 1 334 987, the ammonium carboxylates described in EP-A 1 475 399 or the metal hydroxides and alcoholates described in EP-A 1 475 400. However, these catalysts also lead to powder lacquers having a structured surface and insufficient gloss. Even by using special cross-linking agent components containing uretdione groups, which were produced with 1,12-dodecanediol as chain extender (see e.g. EP-A 1 524 283 and EP-A 1 526 147), and/or special partially crystalline polyester polyols as binder (see e.g. WO 2005/105879 and WO 2005/105880), the problem of the defective optical properties of highly reactive uretdione powder lacquers has not up to now been satisfactorily solved.
Accordingly, the object of the present invention was to provide new cleavage-free PUR powder lacquers based on readily accessible commercial binder components, which harden at low stoving temperatures and correspondingly short stoving times and at the same time produce completely cross-linked lacquer films, which compare to the hitherto known uretdione powder lacquers hardenable at low temperature, and are characterized by significantly improved surface properties, in particular an outstanding flow behavior.
This object was now achieved by providing the uretdione powder lacquers containing monoalcohols described in more detail hereinafter.
The present invention is based on the surprising observation that with cleavage-free PUR powder lacquers consisting of uretdione powder lacquer cross-linking agents, conventional hydroxyfunctional binders, suitable catalysts accelerating the reaction of uretdione groups with hydroxyl groups, and optionally an at least equimolar amount with respect to the carboxyl groups present in the system of a compound reactive to carboxyl groups, coatings having an outstanding flow behavior can already be obtained at temperatures from 110° C. if a defined amount of a solid monoalcohol or a monoalcohol mixture is additionally added to the powder lacquers. The coatings that are thereby obtained exhibit excellent mechanical properties and good solvent resistances, irrespective of the reduction in functionality of the binder caused by the addition of the monoalcohol.